The general forms of most metal detectors are either hand-held battery-operated units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held products include detectors used to locate gold, explosive land mines or ordnance, coins and treasure. Examples of conveyor-mounted units include fine gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to locate buried land mines.
These metal detectors usually, but not necessarily, consist of transmit electronics generating a repeating transmit signal cycle of a fundamental period, which is applied to an inductor, for example a transmit coil, which transmits a resulting varying magnetic field, sometimes referred to as a transmit magnetic field.
These metal detectors may also contain receive electronics that processes a receive signal from a measured receive magnetic field, during one or more receive periods during the repeating transmit signal cycle, to produce an indicator output signal, the indicator output signal at least indicating the presence of at least a metal target within the influence of the transmit magnetic field.
During the processing of the receive signal, the receive signal is either sampled, or synchronously demodulated, to produce one or more target channels, the one or more target channels being further processed to produce the indicator output signal.
Time domain metal detectors typically include pulse-induction (“PI”) or pulse-induction like metal detectors, and rectangular current-pulse metal detectors, wherein the receive processing includes sampling of the receive signal and/or synchronous demodulation over selected periods, which may include gain weighting of the samples or synchronous demodulation periods.
Frequency domain metal detectors typically include single or multi-frequency transmission, or pulse transmission with either sine-wave weighted synchronous demodulation, or unweighted synchronous demodulation with pre synchronous demodulation band-pass and/or low-pass filtering.
Soils (such as magnetic soils) produce relatively large interfering signals compared to typical signals from sought after targets, for example, gold nuggets or landmines To detect such targets in soils, it is important to ensure that the adverse effects of signals from soils towards the receive signal are removed or minimised.
There are known methods to reduce or remove signals from the soils from a receive signal. For example, U.S. Pat. No. 6,559,645 discloses a method whereby a first signal from the environment is recorded and a second signal, which may contain signal from a metallic object, is compared to the first signal, as a means of enhancing signal to noise ratio by reducing the influence of the first signal from the environment to the second signal. However, the measured first signal from the environment may not be representative of the component of the second signal which results from the environment. This is the case with many soils, which contain multiple components which may vary independently. The amounts of the multiple components can vary substantially over a small area of soil, such that recording the signal from the environment in one instance is representative of the signal from the environment in another instance. This is particularly troublesome in the case of isolated rocks which have different amounts of the multiple components than the host soil in which they are embedded. Such rocks are commonly referred to as “hot rocks”.
Advanced methods for reducing or removing signals from soils from a received, signal using synchronous demodulators include those disclosed by Australian Patent Application Nos. 2007304831 and 2009243482.
The invention disclosed herein offers an alternative to the prior art for reducing or removing the effect of signals from soils.